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There are several regulative bodies controlling the abandonment, decommissioning and removal of redundant offshore platforms.In the UK waters this is the remit of the DECC enforcing the Petroleum Act 1998 in consultation with the Paris and Oslo Conventions.
Once the Cessation of Production (COP) Certificate has been issued by the Government, decommissioning can commence.
In the following sections we shall examine current methods of capping the wells and the removal and decommissioning of production platforms from the North Sea; first section dealing with the decommissioning of the risers.
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Decommissioning of Risers and Blowout Preventers
The risers are washed through down into the old reservoir with a degreaser and then blown through with CO2. They can now be disconnected from the blowout preventers and the production headers on the deck. This can be accomplished underwater by explosives or by air lancing. Several surveys have concluded that the use of underwater explosives is detrimental to the aquatic biota, and have recommended the use of air lancing. Disconnection from the production headers requires the removal of flange bolting either by unfastening bolts or burning them off.
Whichever methods are used the risers will require special handling as they will have an internal build up of naturally occurring radioactive material (NORM). These pipes will require specialist treatment at a nuclear power spent fuel facility to remove the accumulations of NORMs.
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Offshore Production Platform Deck Decommissioning and Removal
The decommissioning of the deck processing equipment can now commence. This consists of gas - freeing of all pipelines, vessels, process pumps gas and air compressors.
All diesel and water storage tanks should have been pumped dry at initial shutdown, but these should be rechecked and gas-freed. Power and compressed air should be supplied using temporary power units.
Once again the production headers, HP and LP separators, oil export pumps and associated pipework will all require earmarking for the nuclear facility.
Once this has been completed, the deck can be removed in pieces, usually by modules small enough to safely lift and transport on barges to licensed shoreside facilities.
Specialist equipment such as wireless and radar, lifeboats, lifebuoys, fire extinguishers, power generation units and pedestal cranes, may be reconditioned and sold for reuse.
A good clean-up, and preservation (see article on offshore equipment preservation) along with the maintenance records and the hours run for diesel power generators and cranes will greatly enhance the asking price of these items.
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Offshore Platform Jacket Decommissioning and Removal
The jacket is now left standing and all diesel and water storage tanks in the legs will have been pumped dry and gas-free.
If the jacket is to be removed and shipped or towed ashore then buoyancy tanks will have to be fitted before sailaway and installation. These were used to ensure a steady launch and turning upright of the structure by de-ballasting. They are removed after installation, it may have been an idea to keep them on for decommissioning – comments please.
Anyway new ones are welded in position complete with the necessary operating piping and pumps.
A decision is made to use the air lance or explosives to cut the jacket free from the pile clusters which have anchored it to the seabed for over 25 years.
There is a clause in the decommissioning legislation which allows for some subsea items to be left in situ, such as the export pipelines, the drilling template and a small protrusion of the pile clusters.
The buoyancy tanks are filled with seawater and the piles and cluster guide pipes cut or broke away from the seabed by explosives. Once free, the buoyancy tanks are deballasted and the jacket (hopefully if the engineer’s calcs are good) allowed to float to the surface. From here it can be towed to the licensed onshore decommissioning facility.
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Gas and oil subsea export pipelines from the platform to the shore can be buried in trenches in the sand or covered over with aggregates. These are not usually removed being left in situ after gas-freeing. However some were laid or partially laid on the top of the seabed and if these are left in position they become are a hazard to the fishing industry especially trawlers. The normal procedure for these is to flush them out then leave in situ but, make them trawler net-proof by fitting anti-net fouling equipment over them where they pass through fishing grounds. They must also be marked on shipping/seafloor maps.
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Rigs to Reefs
Whilst the jacket has been supporting the production platform, it has become a habitat for many marine biota- small marine organisms, and Crustaceans readily adopt the legs and cross members as their home and attach themselves in abundance. This brings the fish which also increase in numbers and species. It seems pointless therefore to chop this habitat and remove it (along with its inhabitants) permanently from the seabed depriving the fish from their new found diet.
After ensuring all the oil has been removed from the internal tanks in the legs the structure can be toppled onto its side by cutting through the piles with a compressed air knifing.
This will form a permanent artificial reef, maintaining the present growth of biota and encouraging future fish population.
- UK oil and Gas: Decommissioning
- Offshore Environment: Decommissioning and Removal of Redundant Offshore Structures
- Oil and Gas forum: Decommissioning